PTCs increase resistance as temperature increases due to increased flow. Designed to limit unsafe currents while allowing constant safe current levels, resistance will "reset" automatically when the fault is removed and temperature returns to a safe levels.

The Telecom resettable PTC Series is designed to protect against short-duration high-voltage fault currents (power cross or power induction surge) typically found in telecommunications and networking applications. The 250R, 250S, TRF250, TSL250, TS250, TCF250 and TSV250 series can be used to help telecom networking equipment meet the protection requirements specified in ITU K.20 and K.21. The 600R, TRF600, TR600, TS600 and TSM600 series is designed to be used in applications that need to meet the requirements of GR–1089-CORE and UL60950/ EN60950/IEC60950.

Traditional Fuses vs. PTCs

Fuses and PTCs are both overcurrent protection devices, though each offer their own unique operating characteristics and benefits. Understanding the differences between the two technologies should make the choice in selection easier, depending on the application.

The most obvious difference is that PTCs are automatically resettable whereas traditional Fuses need to be replaced after they they are tripped. Whereas a fuse will completely stop the flow of current (which may be desired in critical applications) after most similar overcurrent event, PTCs continue to enable the equiment to function, except in extreme cases.

Because they reset automatically, many circuit designers choose PTCs in instances where overcurrent events are expected to occur often, and where maintaining low warranty and service costs, constant system uptime, and/or user transparency are at a premium. They are also often chosen in circuits that are difficult to access or in remote locations, were fuse replacement would be difficult.

There are several other operating characteristics to be considered that distinguish PTCs and fuses, and it is also best to test and verify device performance before use within the end application.

PTC Characteristics and Terms

Overcurrent circuit protection can be accomplished with the use of either a traditional fuse or PTC (positive temperature coefficient) device. PTCs are typically used in a wide variety of telecom, computer, consumer electronics, battery and medical electronics product applications where overcurrent events are common and automatic resettability desired.

Littelfuse offers PTCs with the following general forms and features, and come in a variety of sizes and capacities:

Littelfuse PTC Characteristics

Both Polymeric (Positive Temperature Coefficient) PTC and traditional Fuse devices react to heat generated by the excessive current flow in a circuit. A fuse melts open, interrupting the current flow whereas a PTC limits current flow as it rises in temperature, changing from low to high resistance state. In both cases this condition is called "tripping." The graph at right shows the typical response of a PTC to temperature.

Littelfuse Polymer PTCs are made chiefly of high density polyethylene mixed with graphite. During an overcurrent event, a Polymer PTC will heat and expand, which in turn causes the conducting particles to break contact and stop the current.

The general procedure for resetting the device after an overload has occurred is to remove power and allow the device to cool down.

Leakage Current

When a PTC is in a "tripped state" it protects the circuitry by limiting the current flow to a low leakage level. Leakage current can range from less than a hundred milliamps (mA) at rated voltage up to a few hundred milliamps (mA) at lower voltages. Fuses on the other hand completely interrupt the current flow when tripped, and this open circuit results in no leakage current when subjected to an overload current.

Interrupting Rating

PTCs are rated for a maximum short circuit current at rated voltage also known as "breaking capacity" or Imax. This fault current level is the maximum current that the device can withstand safely, keeping in mind that the PTC will not actually interrupt the current flow (see Leakage Current above). A typical Littelfuse PTC short circuit rating is 40A; or for the battery strap PTCs, this value can reach 100A. Fuses do in fact interrupt the current flow in response to the overload and the range of interrupting ratings, vary from tens of amperes (A) up to 10,000 (A) amperes at rated voltage.

Operating Voltage Rating

General use Littelfuse PTCs are not rated above 60V while fuses are rated up to 600V.

Hold Current Rating

The hold (operating) current rating for PTCs can be up to 14A, while the maximum level for fuses can exceed 30A.

Littelfuse PTCs are Recognized under the Component Program of Underwriters Laboratories to UL Standard 1434 for Thermistors. The devices have also been certified under the CSA Component Acceptance Program.

Time-Current Characteristic

Comparing the time-current curves of PTCs to time-current curves of fuses show that the speed of response for a PTC is similar to the time delay of a Littelfuse Slo-Blo® fuse.

Temperature Rerating

The useful upper limit for a PTC is generally 85°C, while the maximum operating temperature for fuses is 125°C.

Ambient temperature effects are in addition to the normal rerating. PTCs hold and trip rating must be rerated when applied at conditions other than room ambient. For example, any rise in ambient temperature will decrease the hold current rating as well as the trip current. A reduction in ambient temperature will increase the trip current as well as the hold current.

The temperature rerating curves in the table below compare PTCs to fuses and illustrate that more rerating is required for a PTC at a given temperature.

PTCs are typically used as circuit protection in applications where sensitive components are at constant risk of damage from overcurrent conditions. The ability of PTCs to reset themselves after exposure to a fault current makes them ideal within circuits that are not easily accessible to a user or technician or where constant uptime is required.

Note: The application summary is for reference only. Determination of suitability for a specific application is the responsibility of the customer.

Typical PTC Circuit Protection Designs

The following are examples of typical circuits using Littelfuse PTCs in combination with other Littelfuse circuit protection devices to provide a comprehensive protection solution. Contact a Littelfuse application expert for design assistance or visit www.littelfuse.com/designcenteror http://www.littelfuse.com/PTCs for additional information. Be sure to to veryify specifications and test device performance before use in the end application.

Consult the Time-Current (T-C) Curve of each PTC series to determine if the selected part will operate within the constraints of your application.

If the device opens too soon, the application may experience nuisance operation. If the device does not open soon enough, the overcurrent may damage downstream components.

To determine the opening time for the chosen device, locate the overload current on the X-axis of the appropriate T-C Curve and follow its line up to its intersection with the curve. At this point read the time tested on the Y-axis. This is the average opening time for that device.

If your overload current falls to the right of the curve the device will open. If the overload current is to the left of the curve, the device will not operate.

Verify ambient operating parameters.

Ensure that the application voltage is less than or equal to the device's rated voltage and that the operating temperature limits are within those specified by the device.

Verify the device's dimensions.

Compare the maximum dimensions of the device to the space available in the application. The dimension of each product is included within each data sheets on the following pages.

This application note addresses the various requirements for protecting the Universal Serial Bus (USB) from overcurrent and over voltage environmental threats.The solutions presented cover both USB 1.1 and the higher speed USB 2.0 circuitry.

This application brief demonstrates how to ensure reliable and safe circuit operation of USB products, protection against voltage and current transients must be carefully considered. Due to the higher data rates of USB 2.0, attention to the electrical characteristics of protection solutions is required.